Not understanding it may be related to one's height (5,4,4)
I believe the answer is:
above ones head
'not' is the definition.
Both the definition and answer are adverbs. Maybe you can see a link between them that I can't see?
'understanding it may be related to one's height' is the wordplay.
I cannot really see how this works, but
'understanding' could be 'head' (eg a head for figures is an understanding of them) and 'head' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
'one's' could be 'ones' and 'ones' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for above ones head that I've seen before include "too hard for an individual to understand" , "Like the firmament" , "Unfathomable" , "beyond understanding?" , "Like a school governor" .)